Growth of SnSe Thin Films for Bulk Photovoltaic Effect Studies

Upon approaching the monolayer limit, SnSe transforms from a centrosymmetric <i>Pnma </i>phase to a polar <i>Pmn</i>2₁phase for an odd number of layers. Layer selective polarity makes SnSe an ideal material to study the impact of structural anisotropy on charge separation and transport such as the bulk photovoltaic effect (BPE). The BPE is a proposed mechanism of charge separation in polar materials which allows for above-bandgap V_OC. There are reports of BPE devices exceeding the Shockley-Queisser limit, but the fundamental limits and underlying mechanisms are not yet established. By directly controlling the polar-nonpolar transition without any confounding independent variables (e.g., temperature), SnSe provides a route to systematically study the BPE mechanisms in 2D materials. However, current methods of SnSe thin film growth, such as mechanical or liquid phase exfoliation, result in films with lateral sizes too small for device fabrication and random crystallographic orientations. Thus, the growth of oriented films with controlled layering is critical to study the BPE in SnSe.<br/><br/>In this study, we explore the phase stability and optical properties of molecular beam epitaxy (MBE) grown SnSe thin films. Thin films of (2 0 0) oriented SnSe were grown by MBE on cleaved (1 0 0) MgO substrates at a deposition temperature of 290 ± 5 °C. No secondary phases or orientations were measured by x-ray diffraction or Raman spectroscopy. The deposition timing was varied to grow films from 5-100 nm thick. We employ UV-vis spectroscopy and photoluminescence to reveal that the bandgap of MBE-grown SnSe increases from 1.4 eV for 100 nm films to 1.9 eV for 5 nm films and the bandgaps are found to be direct for all measured film thicknesses. This is in contrast with previous findings of an indirect bandgap ranging from 0.9-1.4 eV for SnSe thin flakes grown by liquid phase exfoliation (LPE).³Further polarized Raman spectroscopy and photoluminescence spectra with different light polarization directions relative to the polar axes of the films provide further insight into the optical anisotropy of SnSe. By understanding the optical response of SnSe as a function of layering and crystallographic orientation we are taking the first steps towards understanding the mechanisms of BPE in these 2D materials.<br/><br/>(1) Chin, J. R.; Frye, M. B.; Shao-Heng Liu, D.; Hilse, M.; Graham, I. C.; Shallenberger, J.; Wang, K.; Engel-Herbert, R.; Wang, M.; Kyung Shin, Y.; Nayir, N.; Duin, A. C. T. van; Garten, L. M. Self-Limiting Stoichiometry in SnSe Thin Films. <i>Nanoscale</i> <b>2023</b>, <i>15</i> (23), 9973–9984. https://doi.org/10.1039/D3NR00645J.<br/>(2) Minnam Reddy, V. R.; Gedi, S.; Pejjai, B.; Park, C. Perspectives on SnSe-Based Thin Film Solar Cells: A Comprehensive Review. <i>J. Mater. Sci. Mater. Electron.</i> <b>2016</b>, <i>27</i> (6), 5491–5508. https://doi.org/10.1007/s10854-016-4563-9.<br/>(3) Huang, Y.; Li, L.; Lin, Y.-H.; Nan, C.-W. Liquid Exfoliation Few-Layer SnSe Nanosheets with Tunable Band Gap. <i>J. Phys. Chem. C</i> <b>2017</b>, <i>121</i> (32), 17530–17537. https://doi.org/10.1021/acs.jpcc.7b06096.